Syringes of the type discussed herein are known in the art. They comprise a syringe cylinder and a distal end following thereto that is designed as a syringe cone. The distal end comprises a region that is set back in a radial direction, and wherein—seen in axial direction—an edge is formed extending in the circumferential direction. Especially if the body of the syringe is comprised of glass, for manufacturing reasons and/or due to the stresses existing inside the material, it is not possible to configure this edge with an acute angle and/or at a right angle. Such an edge therefore comprises a chamfer and/or has—seen in the longitudinal section—the shape of a ramp. The syringe has an attachment piece that comprises a clamping region. When the attachment piece is separated from the syringe, preferably the clamping region has an inside diameter that is smaller than the outside diameter of the region that is set back in a radial direction at the distal end of the syringe. When the attachment piece is placed onto the syringe in such a way that the clamping region engages with the region that is radially set back, there results an expansion of the clamping region in a radial direction in such a way that holding forces are introduced into the radially set-back region of the syringe. The clamping region comprises a distal edge.
Overall, the attachment piece is held to the body of the syringe by two mechanisms: on the one hand, a frictional grip exists between the clamping region and the region of the distal end of the syringe that is set back in a radial direction; on the other hand, the distal edge of the clamping region is able to engage with the edge that is configured on the region of the distal end of the syringe that is set back in a radial direction, thereby creating a form closure. The interaction of these two mechanisms is intended to prevent easy removal of the attachment piece by pulling it off the syringe.
Disadvantageously, in the syringes that are known in the art the holding forces that are created by the frictional grip, on the one hand, and the form closure, on the other hand, are often insufficient to guarantee the safe operation of the syringe. Upon activation or operation of the syringe, forces are introduced into the attachment piece that may result in a disengagement of the clamped connection and ultimately the separation of the attachment piece from the syringe. With regard to the form closure, it is especially problematic that in known systems the distal edge of the clamping region has an acute angle or a right angle, while the edge that is formed in the region that is set back in a radial direction comprises a chamfer or is designed as a ramp. The result is a line-shaped contact between the two edges that does not allow for the build-up of any useful friction forces. To the contrary, it is possible for the edge of the clamping region to slip off the ramp-shaped edge of the set-back region resulting in the attachment piece being relatively easily pulled off the syringe.
It can be further seen that the known syringe bodies have a relatively large length tolerance. The attachment piece is typically placed upon the body of the syringe in a predetermined position during machine production. Depending on the actual length of the individual syringe body, the attachment piece is brought into a position that it is—seen in an axial direction—arranged closer to or at a greater distance from the syringe cylinder. Since the region that is set back in a radial direction is typically not configured as cylindrical but as slightly tapered, and wherein the outside diameter increases from the distal end toward the syringe cylinder, the result is that a greater expansion of the clamping region occurs the closer the attachment piece is disposed relative to the syringe cylinder. This additional expansion results in increased material stress and possibly over-expansion. This may reduce the elastic clamping forces whereby it becomes easier to pull the attachment piece off the syringe.
Known syringes are often subjected to sterilization after the attachment piece has already been placed on the body of the syringe. During this process, temperatures may be reached that are close to the glass transition temperature of the material from which the attachment piece is made. In this temperature range, in the course of the sterilization, irreversible expansion and/or relaxation of the material of the attachment piece may occur, whereby in turn the clamping and/or holding forces are reduced and the attachment piece can be pulled off the syringe more easily. This is problematic especially if, due to the arrangement of the attachment piece, the clamping region is already expanded considerably by being disposed in relative close proximity of the syringe cylinder. This causes an elevated pre-stressing of the material that may result, in connection with the sterilization temperature, in a relaxation of the material, thereby causing the holding forces to decrease especially.